(1) Field of the Invention
The present invention relates to a measuring device which can directly measure a component of a force acting on a test body, in particular a foil shaped body in a flowing water tunnel, and to a method of using said measuring device.
(2) Description of the Prior Art
One apparatus for measuring forces from flowing air acting on a foil shaped body in a test tunnel is shown in FIG. 1. In this apparatus, the foil shaped body is pivotally mounted in the test tunnel housing. The top pin extends through a flexible connection outside the tunnel housing. The pin outside the housing is connected to a load cell, through linkage, which is used to measure the load or force on the foil. The flexible connection prevents flowing air from escaping which can cause the flow field incident to the foil to become destroyed or distorted. It is also common to see a large plate (i.e., large with respect to the size, chord length, of the foil) attached to the foil ends when the gap between the foil ends and the tunnel housing can cause the flow field to be distorted or destroyed.
Such an apparatus, however, is not usable in water test tunnels. The air tunnel flexible connection provides leak tightness when there is atmospheric pressure inside the tunnel. Such a flexible connection cannot withstand the pressures typically seen inside a water test tunnel (e.g., 50 psi). It is possible to strengthen the flexible connection so it can withstand the pressures involved. This, however, affects the accuracy of the measurements as well as the response time for making the measurement because the connection itself limits the motion of the pin.
This apparatus also cannot be adapted so that it can be easily located in the test tunnel. If the load cell structure was located inside the test tunnel, it would have to be isolated from the test body so the incident flow field would not be distorted or destroyed. As indicated above this is accomplished by affixing a large plate to the end where the flow disturbance can originate. This approach, however, results in a considerable portion of the test tunnel being used by the load cell structure. A small foil is then needed which is undesirable since load forces result and accuracy is degraded.
U.S. Pat. No. 5,036,361 to Roberts illustrates another apparatus for measuring forces from flowing air acting on a foil shaped body. The apparatus comprises a dual strain gage balance system for measuring normal and axial forces and pitching moment of a metric airfoil model imparted by aerodynamic loads applied to the airfoil model during wind tunnel testing. The system includes a pair of non-metric panels rigidly connected to and extending towards each other from opposite sides of the wind tunnel and a pair of strain gage balances, each connected to one of the non-metric panels and to one of the opposite ends of the metric airfoil model for mounting the metric airfoil model between the pair of non-metric panels. Each strain gage balance has a first measuring section mounting the first strain gage bridge and the second strain gage bridge means for measuring normal and pitching moment and a second measuring section mounting third strain gage means for measuring axial force.
The principal deficiency of the Roberts system is that a portion of the exterior surface of the foil must be removed by machining so that a flexure can be attached from the outside. A cover is then placed over the flexure to complete the foil's outer surface. This results in a discontinuity on the foil surface around the edges of the cover. Any such discontinuity may adversely affect both the flow field around the foil and the resulting accuracy of measurements.
U.S. Pat. Nos. 2,865,200 to Gieseler; 2,885,891 to Wilson et al.; 4,372,157 to Caruthers et al.; and 4,845,993 to Horne et al. illustrate various testing devices used in wind tunnel testing systems. U.S. Pat. No. 3,460,383 to Padera illustrates a system for measuring forces on control surfaces or fins of missiles which includes flexure areas on each side of the control surface closely adjacent its shaft. A plurality of strain measuring devices are disposed on the flexure areas so that the distortions of the flexure areas can be accurately translated into indications of forces and moments. All exhibit features which impede installation in water test tunnels.
The problems associated with these prior art measuring systems are overcome by the measuring device of the present invention.